Daily injections of parathyroid hormone (PTH) amino-terminal peptide 1-34, a regiment known as intermittent PTH (iPTH) treatment, stimulate bone formation and reduce the incidence of fracture in postmenopausal women and in elderly men. The increase in bone formation caused by intermittent PTH is mainly due to an increase in osteoblast (OB) number. The prevailing explanations for this are increased osteoblastogenesis, attenuation of pre-OB and OB apopotosis, activation of quiescent lining cells, and signaling in osteocytes. PTH binds to the PTH/PTH-related protein (PTHrP) receptor (PPR or PTH-1R), which is expressed on osteoblasts (OBs), osteocytes and bone marrow (BM) stromal cells (SCs). T Lymphocytes also express PPR, respond to PTH, and stimulate OB differentiation. We have found that the capacity of iPTH to augment bone formation, trabecular bone volume and bone strength is markedly reduced in T cell deficient mice, and restored by adoptive transfer of T cells into T cell deficient mice. These findings suggest that PTH signaling in cells of the osteoblastic lineage is necessary but not sufficient for iPTH to induce maximal bone growth, which indicates that T cells play a previously unrecognized role in the anabolic actions of iPTH. However, the mechanism by which T cells contribute to PTH induced bone anabolism remains to be characterized. Equally enigmatic is whether iPTH targets T cells directly or indirectly. Our preliminary studies suggest that iPTH increases the T cell production of Wnt10b, a Wnt ligand that stimulates osteoblastogenesis by activating Wnt signaling in SCs and OBs. Our initial findings also show that the capacity of iPTH to induce T cell production of Wnt10b is abolished by silencing of the PTH receptor PPR in T cells. Based on these data we hypothesize that iPTH treatment directly stimulates T cells to produce Wnt10b, which leads to increased osteoblastogenesis through activation of the Wnt pathway in SCs and their osteoblastic progeny. Others have demonstrated that PTH stimulates bone anabolism by decreasing the production of Sclerostin, a Wnt inhibitor produced by osteocytes. Based on the published evidence from us and others, we propose the hypothesis that iPTH exerts its anabolic activity by activating Wnt signaling in OBs trough a double effect. The first is an increase in the production of the osteogenic Wnt ligand Wnt10b by BM CD8+ T cells. The second is an increase in the sensitivity of the Wnt receptor complex to Wnt ligands induced by inhibition of the osteocytic production of Sclerostin. The specific Aims to investigate the hypothesis are: Specific Aim 1) To determine the role of CD8+ T cell produced Wnt10b in the mechanism by which iPTH induces bone anabolism. Specific Aim 2) To determine whether direct PPR signaling in T cells is required for T cells to potentiate the anabolic activity of iPTH. Specific Aim 3) To determine the effects on bone anabolism of iPTH treatment in Sost-/- mice and in Sost -/- mice lacking T cell production of Wnt10b. This proposal may reveal a novel relevant role of T cells in the anabolic activity of PTH in bone.